MBE is a useful method for growing multilayer semiconductor devices such as infrared detectors, diode-lasers and the like. As layers of material for such devices are typically grown from high purity elemental components of the material, and are typically grown under ultra high vacuum conditions, for example 10.sup.-8 Torr or less, layer composition may be controlled with high precision. Components of layer material are evaporated from effusion cells which are preferably arranged as isothermal enclosures for material to be melted and evaporated. Layer thickness is controlled by timed deposition at a predetermined flux of evaporated material. The flux is determined, among other factors, by the temperature of the effusion cell.
While favored as a method for development and investigation of experimental and prototype devices MBE has hitherto been considered at least second to metalorganic chemical vapor deposition (MOCVD) as a method of choice for manufacturing quantities of devices. This method, however, while able to provide high growth rates an good layer uniformity, requires the use of highly-toxic, metalorganic precursor materials. This, in turn, requires that extensive safety measures must be provided to protect operators of such MOCVD equipment, and indeed anyone in or around a facility in which it is housed.
It is widely perceived that MBE offers only slow growth rate, is unable to deposit uniform layers over a relatively large area, and requires frequent recharging of cells and maintenance of sophisticated vacuum equipment. This perception is not entirely correct, as MBE apparatus is available including high capacity effusion cells, which can be arranged to deposit relatively uniform layers on a substrate rotated constantly during layer deposition. Capacity of cells is such that they can be used for periods of weeks, or even months, without recharging, allowing a deposition chamber to be maintained under vacuum for that period of time. Substrates are inserted into and removed from the deposition chamber via a vacuum load lock. Typically, however, in these high capacity cells thickness uniformity of growth has been limited to about .+-.2%, and varies significantly as the level of material in the cell is depleted. Flux from such cells as a function of cell temperature also varies with material depletion, requiring that the flux must be remeasured, if not during a device deposition cycle, certainly from one deposition cycle to the next. This can be a particular problem when depositing devices such as vertical cavity surface emitting semiconductor lasers which may include up to a hundred, or even more, individual layers of precisely controlled thickness.
Attempts have been made to improve uniformity and variation of flux distribution with depletion by the use of internal baffles in a cell to shape the flux distribution. The use of baffles however provides for multiple paths by which molecules leaving the surface of molten material may reach a substrate. Molecules which do not proceed directly from the melt surface to the substrate can arrive, indirectly, at the substrate after being reflected or adsorbed and re-emitted one or more times from such baffles, or from an internal wall of the cell via those baffles. The contribution of such indirectly arriving molecules to layer growth is difficult to predict and the proportion of indirectly to directly arriving molecules can be expected to vary significantly with depletion of material in the cell. It is believed that such improvements have been limited to providing instantaneous layer thickness uniformity on the order of about 2%.
There remains a need for further improvement in thickness uniformity of layers deposited in MBE apparatus, as well as a need for an effusion cell which delivers a constant vapor flux independent of the material content of the cell. Improved layer thickness uniformity can increase the number of good devices which are made in a production cycle. Constancy of layer thickness uniformity during a production cycle and from one production cycle to the next can significantly decrease time spent in measuring or calibrating cell flux, thereby shortening production cycles and improving productivity of apparatus.